1. Field of the Invention
The present invention relates to a system and method for selecting equally perceptible colors for use in a discrete reproduction system. In particular, the preferred embodiment of the present invention describes a gray scale printer in which the increments between levels of gray are equally perceptible.
2. Description of Related Art
In a continuous tone reproduction system, all points in the gamut can be produced. Photography is an example of a continuous tone reproduction system. The "gamut" of the printing process is defined as the locus of points in the color space that can be reproduced. In contrast, a discrete reproduction system can only produce a certain set of points. For example, most computer printers are discrete reproduction systems in that only a finite number of picture elements can be stored for reproduction and the elements to be reproduced are stored digitally, so a limited number of points from the gamut are used. Therefore, a computer printer would only need to reproduce a limited number of points in the gamut. There are of course, many types of discrete reproduction systems, such as full color printers, black and white printers, digital cameras, etc.
The present invention is concerned, in general, with the problem of selecting the colors for use in a discrete reproduction system so that the selected colors are maximally distinct from each other. This problem is solved by using equiperceptible steps between colors. "Equiperceptible steps" means herein that the perceived difference between consecutive steps or levels is approximately equal for all steps. The preferred embodiment selects the print density in a gray scale printer such that the levels of gray are equiperceptible.
There are several methods for operating a gray scale printer. One method is to fill in multiple pixels representing a character with black or white on a selected basis to approximate the desired level of gray of the character. This process is known as "dithering" and is quite useful where low resolution is acceptable. Another common method of gray scale printing is to adjust the print density on the paper according to the level of gray desired. The typical gray scale implementation utilizes equal increments in print density to reproduce the corresponding gray scale levels. For example, a four bit gray scale printer can reproduce sixteen levels of gray and the conventional gray scale printer would reproduce sixteen levels of gray (including white and black) with uniform density steps between gray levels.
The difficulty with such conventional gray scale printers is that the human visual apparatus does not respond uniformly to density differences. In fact, the perceived contrast between two steps is related inverse exponentially to their mean density. This causes a given increment in print density between light shades of gray to be far more apparent to the human eye than an identical increment in print density between dark shades. This is readily apparent if observing a gray scale test pattern (where e.g., the sixteen gray levels are printed consecutively). In such a gray scale test pattern there is considerable contrast between the light shades of gray while the contrast between the dark shades are almost indistinguishable. It is, of course, desirable that the contrast between adjacent shades of gray be maximized so that each level of gray can be more easily distinguished from an adjacent level.
One type of gray scale printer uses direct thermal paper, such as used for facsimile transmission, calculators, and computer output. Direct thermal paper imaging consists of:
1. Converting electrical signals to heat.
2. Transferring the heat to the surface of a specially treated paper.
3. Darkening of heated areas as a chemical reaction occurs.
However, gray scale thermal printers have the same deficiency as other types of gray scale printers--i.e. the lack of contrast between the darker shades of gray.
Of course, this problem of lack of contrast between certain color levels is not limited to monochrome printers, and is present in practically any discrete reproduction system.